1. Field of the Invention
The present invention relates to a Radio over Fiber (RoF) link apparatus for transmitting radio frequency up/downlink signals in a mobile communication system. More particularly, the present invention relates to an RoF link apparatus associated with a Time Division Duplex (TDD) scheme.
2. Description of the Related Art
In a mobile communication system, an optical repeater is generally used to expand cell coverage of the system. In particular, optical repeaters employing optical links are broadly used in underground areas such as tunnels, subterranean floors and/or the inside of a building where electric waves do not reach. The optical links are used to transmit radio frequency signals by an optical repeater, and are also used to transmit radio frequency signals to an antenna positioned far away from an access point. Also, the optical repeaters are generally constructed for transmission of radio frequency signals of a wireless mobile communication system, such as a Code Division Multiple Access (CDMA) system or a Wideband CDMA (WCDMA) system.
In mobile communication systems, there are duplex schemes of dividing uplink and a downlink communications so as to transmit radio frequency signals, for example, in a Frequency Division Duplex (FDD) scheme for dividing an uplink and a downlink at different frequencies and a Time Division Duplex (TDD) scheme for dividing an uplink and a downlink at different times.
The FDD scheme has been mainly used in the CDMA system or the WCDMA system, whereas the TDD scheme has been discussed in Wireless Broadband Internet (WiBro) and 4th Generation Mobile Communication (4G), which are new mobile communication systems recently attracting attention.
The TDD scheme is based on the idea that as up/downlink transmission capacity is freely variable and radio frequency up/downlink signals have the same channel characteristic, this variability is regarded as a suitable scheme for a next generation mobile communication system employing multiple antennas. Unlike an existing FDD scheme, the TDD scheme is based on the fact that uplink signals are transmitted at a given (predetermined) time period and downlink signals are transmitted for the duration of the time.
FIG. 1 is a block diagram illustrating a construction of a conventional RoF link apparatus for transmitting/receiving radio frequency up/downlink signals in a TDD mobile communication system in areas where typical means of transmission/reception is limited/obstructed.
Referring to FIG. 1, the existing RoF link apparatus for transmitting/receiving radio frequency up/downlink signals in a TDD mobile communication system includes a center site 120 and a remote site 130 connected to the center site 120 through an optical fiber 140.
As shown in FIG. 1, the center site 120 is connected for communications with an Access Point (AP) 110 (i.e. an upper layer) of a mobile communication system through a Radio Frequency (RF) cable. The center site 120 receives RF signals transmitted from the AP 110, converts the RF signals into optical signals by the electro-optic converter 121, multiplexes the converted signals by the Wavelength Division Multiplexer/De-multiplexer (WDM) 121, and then transmits the multiplexed signals to the remote site 130 of the optical repeater through the optical fiber 140. Moreover, the center site 120 de-multiplexes the optical signals received from the remote 130 site by the WDM 121, converts the de-multiplexed signals into RF signals by the photoelectric converter 123, and transmits the converted signals to the AP 110 through the RF cable.
The remote site 130 of the optical repeater de-multiplexes the optical signals received from the center site 120 by the WDM 131, converts the de-multiplexed optical signals into RF signals by the photoelectric converter 132, and transmits the converted signals to a terminal (not shown) through an antenna 134. Moreover, the remote site 130 converts the RF signals received from the terminal into optical signals by the electro-optic converter 135, multiplexes the converted signals by the WDM 131, and transmits the multiplexed signals to the center site 120 through the optical fiber 140.
In FIG. 1, a switch 133 comprises a switch for setting paths of uplink signals and downlink signals according to the control of the switching timing signals (not shown).
Meanwhile, the RoF link apparatus uses a limited modulation index to ensure linearity and requires high optical output power to transmit the RF signals with high power within the limited modulation index.
Therefore, a maximum noise source affecting performance of the RoF link apparatus is a Relative Intensity Noise (RIN) occurring in a laser used as an electro-optic converter. The RIN is defined by equation (1) below.
                              RIN          ⁡                      (            f            )                          =                              〈                          Δ              ⁢                                                          ⁢                              P                2                                      〉                                P            2                                              (        1        )            
Thus, it is understood from the above equation means that the RIN has a value increasing in proportion to the square of the change in optical power. Therefore, the larger the optical power, the larger the RIN value. The RIN usually has a value within a range of approximately −120 to −150 dB/Hz, which monotonically increases up to the relaxation frequency.
The RIN noise amount can be calculated by equation (2) below.PRIN∝(RIN)·P2in·Rresponsivity·RLoad·BWf  (2)
In Equation (2), RIN represents a noise value, Pin represents optical power, Rresponsivity represents photoelectric efficiency of PD, RLoad represents impedance of the receiver, BW represents a bandwidth, and f represents a frequency.
For example, a RIN value measured after the RIN noise occurring in an electro-optic converter of a center site passes through a photoelectric converter of a remote is calculated as follows. In the following calculation, in order to output thermal noise in dBm, 30 dB is added in the last stage in calculating the thermal noise power.RIN=−150 dB/Hz, Pin=0 dBm, Rresponsivity=1, RLoad=50 ohm, BW=100 KHz
                              P          RIN                =                              [                                          -                150                            +                                                (                                      -                    30                                    )                                ×                2                            +              0              +              17              +              50                        ]                    +          30                                        =                              -            113                    ⁢                                          ⁢                      dBm            /            100                    ⁢                                          ⁢          KHz                    
A result of the above calculation shows a RIN having a small value of −113 dBm/100 KHz. Therefore, even a TDD system does not have a large problem with RIN noise.
However, there may be a probability that the RIN noise has a value, which may be degraded without limitation depending on operation conditions of a laser (e.g. an operation frequency of the laser, a bandwidth, etc). Unlike the 2nd generation mobile communication system operating at a frequency of around 2 GHz, the next generation mobile communication system such as an IMT-advanced system operates at a frequency of around 4 GHz and requires an increased bandwidth of around 100 MHz. Therefore, in consideration of these conditions, the situation may greatly change as compared to 2nd generation.RIN=−120 dB/Hz, Pin=0 dBm, Rresponsivity=1, RLoad=50 ohm, BW=100 MHz
                    PRIN        =                              [                                          -                120                            +                                                (                                      -                    30                                    )                                ×                2                            +              0              +              17              +              80                        ]                    +          30                                        =                              -            53                    ⁢                                          ⁢                      dBm            /            100                    ⁢                                          ⁢          MHz                    
On an assumption that RIN noise having a value of −53 dBm is detected in the photoelectric converter of the remote having the construction shown in FIG. 1, the value of noise, which is generated by crosstalk of a transmission path (Tx path) and input to a reception path (Rx path), is calculated as follows. A result of the calculation shows such that noise would be large enough as to saturate the capacity of a reception side, thereby causing the communication to be impossible.
                    -        53            ⁢                          ⁢      dBm      ⁢                          ⁢              (                              output            ⁢                                                  ⁢            level            ⁢                                                  ⁢            of            ⁢                                                  ⁢            photoelectric            ⁢                                                  ⁢            converter            ⁢                                                  ⁢            in            ⁢                                                  ⁢            Rx            ⁢                                                  ⁢            mode                    ,          embodiment                )              +          80      ⁢                          ⁢      dB      ⁢                          ⁢              (                              total            ⁢                                                  ⁢            RF            ⁢                                                  ⁢            Gain            ⁢                                                  ⁢            of            ⁢                                                  ⁢            HPA                    ,          embodiment                )              -          25      ⁢                          ⁢      dB      ⁢                          ⁢              (                  Switch          ⁢                                          ⁢          isolation                )              =      2    ⁢                  ⁢          dBm      /      100        ⁢                  ⁢    MHz  
As described above with reference to a conventional construction such as shown in FIG. 1, a reception side has a certain noise level, which increases when a high level of noise is input to an input side of an LNA, so that the reception side has a low SNR value and thus has degraded reception sensitivity. Therefore, a TDD next generation mobile communication system which uses a laser having a poor RIN value or requires a broad bandwidth cannot employ an RoF link apparatus with a conventional construction.